The present invention concerns a multi-cylinder dryer for a paper machine, comprising several drying groups situated one after the other, in which drying cylinders are placed one above the other in stacks, preferably in vertical stacks. In at least two subsequent groups, the webs pass from one drying cylinder onto the next drying cylinder by means of a single-wire draw, so that the drying cylinders in the single-wire draw groups are situated outside the loops of their respective drying wires when the web is passed from one cylinder onto the next cylinder over suction rolls while supported by the wire, and onto the drying cylinders of the single-wire groups so that the web enters into direct contact with the heated face of the drying cylinder while pressed by the drying wire over a considerably large sector, most appropriately larger than 180.degree. .
As a rule, the prior-art multi-cylinder dryers for paper machines comprise two lines of steam-heated, large-diameter drying cylinders. These lines are placed one above the other, over which cylinders the web is guided to run in a meandering manner. In the cylinder groups of the prior-art, multi-cylinder dryers, both the single-wire draw and twin-wire draw are used. As a rule, the single-wire draw in which both the drying wire and the web supported by the wire run meandering manner from the lower line of cylinders to the upper line and vice versa, is used in an initial part of drying section, because at that point the web is of higher moisture and lower strength, and by means of the single-wire draw, a closed draw without open transfers can be obtained.
Twin-wire groups in which the web has free draws unsupported by the wires between the lines of cylinders are, as a rule, used in the final end of the drying section, where the web is sufficiently strong so such that the free draws of consequent web and the fluttering occurring therein do not cause excessive breakage of the web. In the case of the single-wire draw on the cylinders, most commonly the lower cylinders which are placed inside the wire loop, the drying wire is in direct contact with the cylinder faces, and the web is thereby outside the wire, which results in lowering of the drying capacity. This is the reason why, when a single-wire draw is employed, several cylinders must be added to an the multi-cylinder dryer.
The present invention relates to improvement over such prior-art dryers in which a particular single-wire draw is used, wherein the cylinders in each drying-wire group are placed outside their respective wire loop so that on all the drying cylinders in a wire group, the web is pressed between the face of the drying cylinder and the drying wire or felt. With respect to these prior art dryers, reference is made to U.S. Pat. Nos. 796,601; 4,483,083; 4,677,762; and to the Valmet Finnish Patent No. 53,333 corresponding to U.S. Pat. No. 3,868,780.
The object of the present invention is to achieve further improvement or development over the drying section described in the Valmet Finnish Patent No. 53,333 corresponding to U.S. Pat. No. 3,868,780, in particular the FIG. 9 of that patent, so that the advantages obtained by means of the prior-art dryer are retained, but the drawbacks occurring therein are avoided. In the drying section illustrated in FIG. 9 of Finnish Patent No. 53,333, the cylinders are placed in vertical, single-wire groups situated one after the other, between which the web has unsupported and free draws. In the present-day, high-speed paper machines, the free draws cause breaks, and therein the drying tension tends to be relaxed, causing detrimental shrinkage of the paper web.
For example, in U.S. Pat. No. 4,677,762, long dry suction boxes are used at the wire transfers between vertical cylinder stacks provided with single-wire draw, by means of which attempts are made to keep the web in contact with the face of the drying wire so that it should not become detached from the wire, and that the drying tensions should not be relaxed into stretching. In order to prevent stretching of the web, it is necessary to use relatively extensive levels of negative pressure, which has the consequence that the faces of the drying wire rub against the suction boxes, which causes, in particular, detrimental wear of the wires.